1. Field of the Invention
This invention relates generally to methods of making carbon composite articles. More particularly, the invention relates to a novel process wherein a porous, rigidized shaped substrate, or preform, is constructed from layers of an interwoven fibrous fabric material which has been heat treated and onto the fibers of which has been thermally fused a very small amount of a carbonaceous binder. To complete the composite product, the substrate is subjected to controlled conditions of temperature and pressure and then is controllably densified by the chemical vapor deposition of pyrolytic material interstitially thereof.
2. Discussion of the Prior Art
Excellent high temperature performance characteristics of carbon composites in structural, frictional, ablative and thermal insulation applications has caused an ever expanding demand for such materials. Accordingly considerable effort has been extended in recent years in developing new techniques for the large scale production of such materials for use in nuclear, aerospace, aircraft and industrial fields.
In producing carbon composite products, substrates or preforms, made up of carbon fibrous materials are first constructed. Two basic methods are typically employed to produce the substrates. In accordance with one method, sometimes called the resin bonding method, the fibrous substrate materials are bonded together using substantial amounts of carbonizable binder such as phenolic resin or the like. The methods described in the patent to Bickerdike et al, U.S. Pat. No. 3,233,014 is exemplary of the resin bonding method. In accordance with the second method, often called the Chemical Vapor Deposition or C.V.D. method, the fibrous substrate materials are bonded together by the interstitial deposition of pyrolytic carbon using known chemical vapor deposition (C.V.D.) techniques. The patent to Bauer, U.S. Pat. No. 3,895,084, clearly describes the C.V.D. method. Also pertinent to this method is the British patent to Williams, U.S. Pat. No. 1,455,891.
The resin, or carbonaceous binder method has certain recognized advantages. For example, using this method, durable, high density structurally stable preforms can readily be produced using conventional molding and press bonding techniques. The preforms formed by this method can conveniently be shaped and are easily handleable. Drawbacks of the resin binder method, however, include the fact that the method is relatively expensive requiring several preparatory processing steps including pre-impregnation of the substrate materials, "B" staging and binder carbonization. Additionally, difficulties have been experienced using this method in achieving acceptable compatability between fiber and binder processing shrinkages, and in routinely producing preforms which retain acceptable part integrity during the various substrate processing steps without delaminating or micro-cracking. In a similar vein, the relatively high quantities of carbonizable binder necessary to produce an acceptable carbonizable substrate frequently pose debulking, outgassing and dimensional stability problems during carbonization of the binder. A frequent objective of the resin bonding method is to totally encapsulate the individual fibers of the substrate in an attempt to eliminate undesirable voids in the substrate.
The C.V.D. method, wherein substrate bonding is accomplished by depositing onto the fibers of the substrate carbon resulting from dissociating methane or other carbon bearing source gasses, also has several recognized advantages. For example, this technique produces a substrate having maximum open porosity so as to permit precisely controllably partial or complete substrate densification. Additionally, inherent in the method is the fact that each fiber of the substrate is uniformly coated with the deposited material rendering it substantially impermeable and unusually resistant to corrosion even at high temperatures. Further, the thickness of the coating on the fibers themselves and at the fiber cross-over points can be precisely regulated so as to achieve the desired substrate rigidity.
A major disadvantage of the C.V.D. method is that some form of expensive and often bulky shaping fixture is required to hold the substrate materials in the desired configuration until sufficient pyrolytic carbon has been deposited to rigidize the fibrous structure. Such hardware is expensive, reduces furnace productivity substantially since it occupies a significant portion of the severely limited furnace processing volume, and presents significant assembly and disassembly problems.
As will become apparent from the discussion which follows, the method of the present invention permits realization of the advantages of both the resin and C.V.D. methods while substantially avoiding the disadvantages of each method.
A method devised to attempt to overcome the problem of clogging the intersticies of the substrate with resin is described in co-pending U.S. application Ser. No. 047,158, now abandoned, wherein the present inventor is named as a co-inventor. The drawbacks of the method described in said application relate to difficulties encountered in precisely controlling the amount of resin used as a temporary bonding agent and in handling the fabrics to which the resin is applied. In the process of the previously described invention the resin impregnated fabric is fragile and difficult to handle and cut. Additionally, during handling, the resin applied to the material tends to powder and spall making precise control of the amount of resin contained in the shaped substrate impossible. The method of the present application uniquely and effectively solves this problem in a way which enables large volume production of the starting substrate material.
In addition to the prior art identified in the preceeding paragraphs, applicant is aware of a printed publication entitled "Development of High Modulus Carbon Fiber Tape Composites" published by United Technology Laboratories which describes methods of making composite articles using carbon fiber tape.